Most Hard‐X‐Ray–Selected Quasars in the Chandra Deep Fields are Obscured

Abstract

Measuring the population of obscured quasars is one of the key issues in understanding the evolution of active galactic nuclei (AGNs). With a redshift completeness of 99%, the X-ray sources detected in the Chandra Deep Field South (CDF-S) provide the best sample for this issue. In this paper, we study the population of obscured quasars in CDF-S by choosing the 4-7 keV selected sample, which is less biased by the intrinsic X-ray absorption. The 4-7 keV band-selected samples also filter out most of the X-ray-faint sources with too few counts, for which the measurements of NH and LX have very large uncertainties. Simply adopting the best-fit L2-10 keV and NH, we find that 71% ? 19% (20 out of 28) of the quasars (with intrinsic L2-10 keV > 1044 ergs s-1) are obscured with NH > 1022 cm-2. Taking into account the uncertainties in the measurements of both NH and LX, conservative lower and upper limits for the fraction are 54% (13 out 24) and 84% (31 out 37). In the Chandra Deep Field North, the number is 29%; however, this is mainly due to the redshift incompleteness. We estimate a fraction of ~50%-63% after correcting for the redshift incompleteness with a straightforward approach. Our results robustly confirm the existence of a large population of obscured quasars.